Designing a protocol for the first time

Keith M wrote:

Snip...

> Xmodem is considerably easier to implement than Zmodem.  I implemented 
> Xmodem when I was about 10 or 11 years old, first in Basic, and then in 
> C.  

Brings back old memories.  My first product had X-Modem
download capability and I wrote and debugged it in assembly
language on a Saturday afternoon.  It was exquisitely
ugly with software timing loops and no crc, but it
worked for years.  I finally got sick of it and completely
rewrote it with crc.

Keith M wrote:
> CBFalconer wrote:
> 
>> If you are going to consider X-modem, try Z-modem.  Much better
>> error control, speed, etc.  Uses the same transmission paths.
>>
> 
> Sorry to pick on your idea CBF.
> 
> Xmodem is considerably easier to implement than Zmodem.  I implemented 
> Xmodem when I was about 10 or 11 years old, first in Basic, and then in 
> C.  

That's the sort of thing I can deal with!

>I remember Zmodem when it was first released, and it was great.  Big 
> improvement over Xmodem and Ymodem, lots of extra features and 
> functionality.  But it was also a lot more complex.  Sliding windows, 
> automatic recovery, big checksums, transfer of filenames, etc etc.
> 
> I'm not necessarily advocating anything for your particular application, 
> just wanted to point out that using Zmodem might not be a slam dunk.

I wouldn't have considered Zmodem for my app.

> Oh, and regarding FTDI chips.  Their chips, drivers, and support are all 
> top notch.  I'm using their FT232BM chip at 2mbps, and it works 
> reliably.  I started using their VCP drivers, which work very well.  I 
> eventually switched to using their D2XX .dll drivers, because it offers 
> more fine-grain control.
> 
> Be sure to download the datasheets, app notes, and tips on their 
> website, especially when they talk about latency, baud rates, etc.

Thanks for the input to the thread!


-- 
_____________________
Christopher R. Carlen
crobc@bogus-remove-me.sbcglobal.net
SuSE 9.1 Linux 2.6.5

Jim Stewart wrote:

> Brings back old memories.  My first product had X-Modem
> download capability and I wrote and debugged it in assembly
> language on a Saturday afternoon.  It was exquisitely
> ugly with software timing loops and no crc, but it
> worked for years.  I finally got sick of it and completely
> rewrote it with crc.

Jim,

Sorry to bring the thread back from the dead -- just saw your reply now.

Yeah, old memories indeed.  I co-wrote some (unpopular, basically 
unknown) BBS software years ago as a kid, and one of my tasks was 
implementing the file upload/download feature.  I remember finding 
XMODEM.TXT(the xmodem specs) on some bulletin board, and being very 
excited about it.  At the time, however, I didn't have enough RAM to 
store it, and I couldn't write real time to my tape drive.  So what I 
did was attach my modem to my printer directly (daisy wheel 130 column 
25cps), and lo-and-behold, I managed to get a printed copy of it!  300 
baud is +/- 25cps, so a little flow control, and I was home free. 
Ingenious for not being yet in my teenage years! :)

Anyways, I ended up not understanding how the 8-bit checksum worked (I 
think it was just check=check+newbyte mod 256.), and so I picked up the 
phone and called the phone number at the top of the page.(of course!) 
Turned out to be either Ward Christensen or Chuck Forsberg at some 
university.  I remember him telling me, "it's all in the spec" but I 
didn't know enough to understand most of what was in there.  He provided 
a couple quick pointers and shoo'd me away back to my paper route.

Ahh well.

Keith